force. However, the Republicans remained firmly entrenched along the Pingarrón heights on the eastern bank and continued to plaster the Nationalist bridgeheads with artillery fire. Barrón's brigade was held up by the Garibaldi Battalion, which held the high ground near Arganda. Late in the day, units of XI International Brigade held off a Nationalist push onto the Arganda–Colmenar road. The Republicans then counterattacked twice with Soviet T-26 tanks, which were beaten off with artillery fire from Nationalist batteries dug in on La Marañosa, but they served to hold up further Nationalist advances. When Junkers of the Condor Legion appeared overhead in support of the Nationalists, Republican planes shot them down and took control of the skies. Until 13 February the Republican air force, largely composed of Soviet machines and pilots, maintained air supremacy. However, they were challenged by the arrival of more Italian and Spanish nationalist aircraft and a large scale dogfight was fought over Arganda, and they suffered heavy losses from German 88 mm guns while undertaking ground attack missions. Suicide Hill The Nationalists brought their reserves forward and on 12 February opened a powerful attack in the direction of Morata. Asensio's troops took the Pingarrón hills and assaulted the Pajares heights to the north. This struggle for the high ground east of the Jarama would see some of the most bitter fighting of the battle. The Republican XI International Brigade and 17 Brigade defending the Pajares found itself outmanned and outgunned. Nationalist artillery massed on the heights of Pingarrón and pummeled the defenders, but they managed to hold. Meanwhile, along the San Martin–Morata Road, the newly formed XV International Brigade, consisting of a British Battalion, the Balkan Dimitrov Battalion and the Irish, and the Franco-Belgian Sixth of February Battalion, had been hurriedly put into the line to help stem the tide of the Saenz de Buruaga's brigade. Heavy fighting followed and the Nationalist advance was blunted. A furious and confused fight followed in which the British Battalion lost poet Christopher Caudwell and 375 of their 600 men, including almost every officer, including the battalion commissar and Captain Tom Wintringham, in gaining and then holding and finally retreating from a position they named "Suicide Hill". On their right, though, the Franco-Belgians were forced to withdraw suddenly and in the ensuing confusion the British Battalion's machine-gun company was captured. To their right, the Dimitrov Battalion fought a desperate defensive action